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Most MVVM examples I have worked through have had the Model implement INotifyPropertyChanged, but in Josh Smith's CommandSink example the ViewModel implements INotifyPropertyChanged.
I'm still cognitively putting together the MVVM concepts, so I don't know if:
INotifyPropertyChanged in the ViewModel to get CommandSink to workINotifyPropertyChanged and this is just a mistake which would be corrected if this were developed from a code example to an applicationWhat have been others' experiences on MVVM projects you have worked on?
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The purpose of ViewModel is to encapsulate the data for a UI controller to let the data survive configuration changes. For information about how to load, persist, and manage data across configuration changes, see Saving UI States.
The INotifyPropertyChanged interface is used to notify clients, typically binding clients, that a property value has changed. For example, consider a Person object with a property called FirstName .
Models are just the plain data, and a ViewModel is something that acts like a padding in between the two, that it should get information from the Model and pass it onto the View, and the View should know how to present it.
If you want your Models to alert the ViewModels of changes, they should implement INotifyPropertyChanged, and the ViewModels should subscribe to receive PropertyChange notifications. But typically this is only needed if more than one object will be making changes to the Model's data, which is not usually the case.
I strongly disagree with the concept that the Model should not implement the INotifyPropertyChanged. This interface is not UI specific! It simply informs of a change. Indeed, WPF heavily uses this to identify changes, but that doesn't mean it is an UI interface. I would compare it to the following comment: "A tire is a car accessory". Sure it is, but bikes, buses, etc. also use it. In summary, do not take that interface as an UI thing.
Having said that, it doesn't necessarily mean I believe that the Model should be providing notifications. In fact, as a rule of thumb, the model should not implement this interface, unless it is necessary. In most cases where no server data is pushed to the client app, the model can be stale. But if listening to financial market data, then I do not see why the model cannot implement the interface. As an example, what if I have non-UI logic such as a service that when it receives a Bid or Ask price for a given value it issues an alert (ex. through an email) or places an order? This could be a possible clean solution.
However, there are different ways of achieving things, but I would always argue in favor of simplicity and avoid redundancy.
What is better? Defining events on a collection or property changes on the view model and propagating it to the model or having the view intrinsically update the model (through the view model)?
The bottom line whenever you see someone claiming that "you can't do this or that" it is a sign they do not know what they are talking about.
It really depends on your case and in fact MVVM is a framework with lots of issues and I am yet to see a common implementation of MVVM across the board.
I wish I had more time to explain the many flavors of MVVM and some solutions to common problems - mostly provided by other developers, but I guess I will have to do it another time.
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